With a dramatic increase in communication demand in recent years, the scope of application of a photonic network is becoming increasingly wider and also the capacity and functionality of networks are increasing. A semiconductor optical amplifier (SOA) has, when compared with an optical fiber amplifier module, a very simple configuration. Thus, the SOA has been examined for application to a network as a small and low power consumption optical amplifier. Moreover, the SOA has high non-linearity in optical gain and is promising as an active element used for various kinds of signal processing such as optical signal regeneration and optical wavelength conversion. A structure in which an antireflection film is formed on both surface on which signal light is incident and a surface from which the signal light is emitted is known as such an SOA (see, for example, Japanese Laid-open Patent Publication No. 2007-67222).
When an SOA is used as an optical amplifier or an optical signal processing element in a photonic network, high-speed gain response is desired. However, the SOA has a gain saturation phenomenon in which the optical gain decreases with increasing optical output from the SOA. The gain saturation phenomenon notably occurs generally in an optical output range of +0 dBm to +10 dBm. For example, when an intensity modulated signal light having a modulation rate of 10 Gbps or more is amplified up to the power regime where the gain of an SOA saturates, a pattern effect due to the gain saturation phenomenon appears and the waveform of an output signal light is deteriorated significantly, leading to lower communication quality. Waveform deterioration of signal light due to the pattern effect is a phenomenon that occurs because a modulation rate of the signal light and a gain response speed in the SOA are at a comparable speed.
Incidentally, it is known that the gain response speed in an SOA is determined by the lifetime of carriers (electrons or holes) contributing to an optical gain in an active layer of the SOA. The lifetime of carriers in an active layer is generally determined by active layer materials and doping conditions of impurities, and it is difficult to achieve a significant speedup by changing the active layer materials or optimizing the structure of an SOA. Thus, a method of external optical injection and a method of an optical self-saturation effect via amplified spontaneous emission (ASE) light are known as methods to increase the gain response speed of an SOA (see, for example, F. Girardin, G. Guekos, and A. Houbavlis, “Gain Recovery of Bulk Semiconductor Optical Amplifiers”, IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 10, No. 6, JUNE 1998).
However, according to the method of external optical injection, it is necessary to set up an additional light source outside the SOA, and the structure thereof becomes more complex, making the method inappropriate for miniaturization. According to the method of an optical self-saturation effect via ASE light, an external light source or the like is not needed, but it is necessary to sufficiently increase intensity of ASE light to saturate the active layer of an SOA in advance. Thus, it is necessary to set the length of SOA chip longer than that of an ordinary one. However, it also causes a large increase of the total power consumption of SOA because the drive current of SOA increases in propotion to its length under constant drive current density.